Fuel savings and reducing emissions of certain gases are a major issue for airlines and therefore for pilots. The new taxes on emissions are prompting the airlines to take this issue into consideration. The problem faced is how to assist the crew in saving on fuel and minimizing the emissions by offering it a vertical cruising trajectory that is optimized through several altitude changes. Since the aerodynamic efficiency of aeroplanes is better at altitude, there is generally an interest in flying as high as possible. However, since a heavy aeroplane exhibits worse performance levels when climbing, the optimum altitude produces a trade-off between these two conflicting aspects: it therefore increases during the flight, as the aeroplane becomes lighter. A number of operational solutions currently exist for approaching this optimum altitude.
It should be recalled that a flight plan comprises a succession of geographic latitude and longitude points, called waypoint (WP). From the waypoints, a lateral trajectory is generated by the FMS, and from this lateral trajectory, a vertical trajectory, called vertical profile, is computed by the FMS, taking into account any altitude, speed, time or other such constraints.
The complete vertical profile is broken down into three phases:
a climb phase from the departure airport to a first altitude level,
a succession of altitude levels to be reached and of associated altitude change points, called “steps”, generally identified on curvilinear abscissa along the vertical profile, for example in relation to the remaining distance to be travelled to arrive at the destination (“distance to destination”). This phase constitutes the cruising phase,
a descent phase from the last altitude level to the arrival airport.
A first solution is a determination of flight level changes filed in the flight plan by the airlines. To this end, the airlines generally propose, in the flight plans determined in the preparation phase, the optimum levels in the cruising phase, as well as the geographic waypoints (WP) at which these level changes must be initiated, commonly called steps. The disadvantage is that the pilots do not generally appreciate this solution because the steps are forced to be performed at geographic points which are unrelated to the vertical flight plan. This does not necessarily constitute an optimum solution because these are purely geographical points which have no meaning for the vertical trajectory.
A second solution is to indicate the value of the optimum flight level during the flight. A computer onboard the aeroplane determines in real time the instantaneous optimum aerodynamic level as a function of the weight of the aeroplane and the pilot can follow the changes of this level to modify its vertical trajectory. This solution does not offer the possibility of an advance planning of the trajectory followed by the aeroplane to the end of the cruising phase. Moreover, it supplies an altitude based only on the aerodynamic efficiency of the aeroplane and the instantaneous wind and is not able to take account of the changes in the wind along the trajectory, which can penalise actual consumption.
A third solution is an embedded function called “Optimum Step” (OPT STEP) currently implemented in certain onboard computers for an aircraft. This function is used during the flight and assists the crew in terms of optimization of fuel consumption during the cruising phase. This function computes the optimum altitude changes (optimum steps) where there is a change from one cruising altitude to another. This change can be made without preference to a lower or higher altitude. The computed step can take place at a specific waypoint of the flight plane (geographic step) or at a point computed by the FMS which does not correspond to a waypoint.
The computation is performed by successive tests for different level change abscissa values. An estimation of the direct operational costs is computed for each abscissa tested, and the value retained is the one for which the cost is lowest.
The new fuel consumption is computed for a single level, corresponding to a step between the current level and an optimum level entered manually by the pilot. The optimization consists only in choosing the moment at which the transition between the current level and the level requested by the pilot has to be made. The computation of the new consumption is displayed to the pilot, who must mentally make the comparison with the consumption without optimization. A drawback of this OPT STEP function is that the optimization is performed on the assumption that the aeroplane remains on the new level until the end of the flight. This function therefore represents a short range optimization of the consumption and with low guarantee.
The graphical interfaces of the embedded systems of FMS type include a page which makes it possible to display the computed steps by giving the cost/gain information associated with the step independently of its type.
The display of the steps respects their chronological order of appearance along the flight plan, the information on the distance remaining before the step is displayed for each waypoint belonging to the active flight plan. When the point of initialization of a step approaches, a “STEP AHEAD” message is displayed on the page in order to warn the crew.
A disadvantage of this solution is that the crew performs steps but has no knowledge of the total gains that it will be able to obtain at the end of cruising. Furthermore, only one optimum step at a time can be taken into account in the computation, which does not make it possible to optimumly anticipate the next altitude changes.
An improvement of the “Optimum step” function is described in the document U.S. Pat. No. 5,574,647.
The proposed method consists in determining an altitude profile that minimizes the direct operational costs. For this, the optimum altitude level (commonly called flight level) that minimizes the direct operational costs is computed instantaneously, that is to say without taking into account what will happen later. This instantaneous optimum flight level is determined using meteorological data, the weight of the aeroplane, and its speed. To this is added a filtering technique so as to inhibit step phases within an excessively short level.
According to this document, the online determination of the optimum cruising altitude (given a speed) cannot be performed, in as much as a plurality of minima can exist, which would entail the use of enumerative methods that are costly in terms of computation time.
This method does not optimize the speed along the cruising phase. The latter is pre-computed in the form of “Econ-Speeds” charts.
Furthermore, this method is based on an optimization at each instant of the altitude, which does not necessarily correspond to an optimization of all of the cruising trajectory.